5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivatives

ABSTRACT

A 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative of the formula (I):                    
     wherein Het is oxadiazolyl, R 1  is H, lower alkyl, cyclo-lower alkyl, trifluoromethyl, lower alkenyl, lower alkynyl, lower alkoxy, lower alkoxy-lower alkyl, hydroxy-lower alkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaromatic group, and R 2  is H, lower alkyl, cyclo-lower alkyl, cyclo-lower alkylmethyl, lower alkenyl, cyclo-lower alkenyl, lower alkynyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaromatic group, or a pharmaceutically acceptable acid addition salt thereof, which has high selective affinity to benzodiazepine receptor and is useful particularly as a benzodiazepine inverse agonist, for example, as psychoanaleptic drug or a drug for the treatment of dysmnesia in senile dementia or Alzheimer&#39;s disease.

This application is a 371 of PCT/JP96/03134 filed Jul. 14, 1998.

TECHNICAL FIELD

The present invention relates to novel a 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative or a pharmaceutically acceptable acid addition salt thereof which is useful as a medicament, and use thereof as a medicament, and further intermediates for preparing the same.

BACKGROUND ART

Benzodiazepine (BZP) compounds, such as diazepam which is a representative compound, have anxiolytic activity and hence have been developed as anxiolytic drugs, but they have also anticonvulsant, sedative and hypnotic activities and hence these compounds have been used in wide clinical fields such as (1) anxiolytic drug, (2) sedative (hypnotic) drug, (3) muscular relaxant, and (4) antiepileptic drug.

BZP compounds have mainly the pharmacological activities such as (1) acclimating activity, (2) hypnotic activity, (3) central muscle relaxant activity, (4) anti-convulsant activity. It is understood that these activities are not exhibited by independent mechanism separately, but are induced by closely related neuropharmacological mechanisms.

Since late 1970s, with progress of pharmacological investigation of BZP compounds, there have been found two footings for clarifying the mechanism of exhibiting the activities thereof, one being a phenomenon of increasing γ-aminobutyric acid agonistic (GABAergic) neurotransmittant mechanism of the central nervous system by the BZP drugs, and another being new finding of BZP specific binding site (BZP receptor) and proving of a mechanism of the functional connection between the brain BZP receptor and GABA receptor. As the result of such investigation, it has almost been established that the GABAergic neurotransmittant mechanism participates in the pharmacological activities of BZP compounds.

Administration of BZP compounds induces side effects such as ataxia, hypnosis, muscle relaxation or lowering of ability of cognition or reflex movement and further formation of resistance and dependence to the drugs, and hence, there are many problems to be improved in the BZP compounds. Studies have been made on non-BZP compounds which have a different chemical structure from BZP compounds but have similar functions in the activation mechanisms. Those compounds including such non-BZP compounds are called as benzodiazepine receptor agonistic drugs. As the non-BZP compounds, there are known, for example, the compounds having the chemical formulae (A), (B) and (C) as shown below.

The compounds having the formulae (A) and (B) are disclosed in Journal of Medicinal Chemistry, vol. 34, p. 2060 (1991).

wherein R_(a) is a hydrogen atom, R_(b)-R_(d) are a methyl group, etc., and R_(e) is a methoxy group, etc.

The compounds of the formula (C) are disclosed in EP-A2-0588500.

wherein Het is an oxadiazolyl group, R¹ is a benzyl group, etc. and R² is a methoxy group, etc.

However, with progress of investigation, there has been found a certain compound among the non-BZP compounds, which has similar high selective affinity to the benzodiazepine (BZP) receptor but has entirely inverse activities [Braestrup, C. et al., Neuropharmacol., 22, pp.1451-1457 (1983)]. When these compounds are administered, they exhibit pharmacological activities such as convulsion increasing activity, anxiety inducing activity, muscle hypertonia. Accordingly, the old BZP compounds which have hitherto been used as anxiolytic drugs are defined as an agonist, and the compounds having thus inverse activities are defined as an inverse agonist.

Since these inverse agonists have been found, intensive studies have been done on the correlation between the modifying (binding) manner and the pharmacological activities of the compounds which bind (exhibit affinity) to the BZP receptor. According to these studies, it has been found that the BZP receptor is present between the GABA receptor (an depressive neurotransmittant) and a chloride ion channel and is a molecular unit to form a complex. The GABA receptor includes an ion channel type GABA_(A) receptor and a metabolism controlling type GABA_(B) receptor, and the GABA_(A) receptor forms a complex with a BZP receptor and the Cl ion channel. The compounds to be bound to the BZP receptor are now classified into an agonist (further subsequently classified into a full agonist and a partial agonist), an inverse agonist (further subsequently classified into a full inverse agonist and a partial inverse agonist) and an antagonist.

The agonist binds selectively to the BZP compounds and thereby acts increasing coupling of the GABA receptor and the Cl ion channel and increases flowing of Cl ion into cells owing to increase of open-close frequency of the Cl ion channel and then stimulates the cell activities owing to decrease of negative electric charge (increases cell stimulation). It is said that the antagonist does not change the coupling function thereof but inhibits binding of the agonist or inverse agonist to the BZP receptor.

There are many method for checking the manner of binding of the compounds to the benzodiazepine receptor, and one of the known methods is a TBPS binding assay. As mentioned hereinbefore, the GABA_(A) receptor forms a complex with a BZP compound receptor and the Cl ion channel, and it is known that a neurosteroid receptor is present on the GABA_(A) receptor membrane and a TBPS(t-butylbicyclophosphonothionate) bond recognizing site is located around the Cl ion channel. The function of GABA to the nervous system is modified and controlled by controlling of the opening of the Cl ion channel and transmission of Cl ion into cells within the GABA_(A) receptor complex molecule under complicated mutual effects. By checking many drugs which act directly or indirectly on the function of the GABA_(A) receptor complex, it is known that there is a good inverse correlation between the test data of TBSP binding and the test data of Cl ion uptake into cells. For instance, the uptake of Cl ion into cells is decreased by GABA_(A) receptor agonists (e.g. Muscimol), neurosteroid receptor agonists, diazepam which is the representative benzodiazepaine receptor agonist, or chlonazepam which is a partial agonist, and is increased by benzodiazepine receptor inverse agonist [e.g. DMCM (methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate)] and a partial inverse agonist [e.g. FG7142 (N-methyl-β-carboline-3-carboxamide)]. Accordingly, the TBPS binding assay is useful for clarifying the GABA_(A) receptor function, the in vitro biochemical screening of the drugs acting via allosteric binding site of bezodiazepine drugs, GABA_(A) receptor complex, etc., and the acting mechanisms of the drugs.

Most of the old BZP compounds such as the compounds of the formulae (A), (B) and (C) have agonistic properties. On the contrary, some compounds having inverse agonistic properties are known, for example, the compounds of the following formulae (D) and (E) (DMCM and FG7142):

The DMCM and FG7142 are disclosed in Colin R. Gardner, Drugs of the Future, vol. 14, pp. 51-67 (1987).

In addition, many investigations have also been made on the correlation between the binding manner to the benzodiazepine receptor and the pharmacological activities of the compounds. As mentioned above, the BZP agonists have been used as anxiolytic drug, hypnotic disorder curing agent (sleep inducing drug) or antiepileptic drug, but it is known that in addition to these activities, they have also an amnestic activity in animals including also human being. Accordingly, BZP inverse agonists are expected to have activities inverse to the amnesia inducing activity, that is, anti-amnestic activity, psychoanaleptic activity. Moreover, it is known that the activity of acetylcholine, which has an important relation to cognition function, is decreased by the BZP agonists and is increased by the BZP inverse agonists, and hence the BZP inverse agonists are expected to exhibit cognition enhancing activity. Thus, it has been expected that the BZP inverse agonists may be useful as psychoanaleptic drug and a drug for treating dysmnesia in senile dementia, cerebrovascular and Alzheimer's dementia.

There is no report as to the compounds of the present invention which have the formula (I) described hereinafter and have high selective affinity to a benzodiazepine receptor and particularly acts as a BZP inverse agonist.

DISCLOSURE OF THE INVENTION

This invention provides a novel 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative having the following formula (I) or a pharmaceutically acceptable acid addition salt thereof which has high selective affinity to a benzodiazepine receptor, and a use thereof as a medicament.

wherein Het is an oxadiazolyl group,

R¹ is a hydrogen atom, a lower alkyl group, a cyclo-lower alkyl group, a trifluoromethyl group, a lower alkenyl group, a lower alkynyl group, a lower alkoxy group, a lower alkoxy-lower alkyl group, a hydroxy-lower alkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group, and

R² is a hydrogen atom, a lower alkyl group, a cyclo-lower alkyl group, a cyclo-lower alkylmethyl group, a lower alkenyl group, a cyclo-lower alkenyl group, a lower alkynyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group.

This invention further provides novel 1,6-naphthyridin-2(1H)-one derivatives of the following formula (I′) which are useful as an intermediate for preparing 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivatives of the above formula (I) useful as a medicament.

wherein R is a cyano group, a carbamoyl group, a carboxyl group, a lower alkoxycarbonyl group, or a substituted or unsubstituted benzyloxycarbonyl group, and

R² is a lower alkyl group, a cyclo-lower alkyl group, a lower alkenyl group, a cyclo-lower alkenyl group, a lower alkynyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group, provided that R² is not a methyl group or a pyridyl group.

During the intensive studies of non-benzodiazepine compounds having affinity to an intracerebral benzodiazepine receptor, the present inventors have found that the 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivatives of the above formula (I) have a high selective affinity to a benzodiazepine (BZP) receptor and hence are useful as a benzodiazepine receptor agonistic drug and further that those compounds include a compound having a BZP agonistic activity and a compound having a BZP inverse agonistic activity which depend on the kinds of combination of the substituents R¹ and R².

Among the compounds of this invention, preferred compounds are the compounds of the formula (I) wherein R¹ is a C₁-C₃ alkyl group, a C₃-C₄ cycloalkyl group, or a C₂-C₃ alkenyl group, and R² is a hydrogen atom, a C₁-C₄ alkyl group, a C₃-C₆ cycloalkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group.

More preferred compounds are the compounds of the formula (I) wherein R¹ is a C₁-C₃ alkyl group or a C₃-C₄ cycloalkyl group, and R² is a hydrogen atom, a C₁-C₃ alkyl group, a C₃-C₄ cycloalkyl group, a substituted or unsubstituted phenyl group, or a substituted or unsubstituted heteroaromatic group.

Further preferred compounds are the following compounds.

3-(5-Ethyl-1,2,4-oxadiazol-3-yl)-5-(2-methylcyclopropyl)-1,6-naphthyridin-2(1H)-one,

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(2-methylphenyl)-1,6-naphthyridin-2(1H)-one,

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one,

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(4-methoxyphenyl)-1,6-naphthyridin-2(1H)-one,

3-(5-Ethyl-1,2,4-oxadiazol-3-yl)-5-(2-thienyl)-1,6-naphthyridin-2(1H)-one,

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(4-pyridyl)-1,6-naphthyridin-2(1H)-one,

3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-methyl-1,6-naphthyridin-2(1H)-one,

3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-(3-fluorophenyl)-1,6-naphthyridin-2(1H)-one,

3-(3-Methyl-1,2,4-oxadiazol-5-yl)-5-(3-methylphenyl)-1,6-naphthyridin-2(1H)-one,

3-(3-Methyl-1,2,4-oxadiazol-5-yl)-5-(3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one,

3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-(4-methoxyphenyl)-1,6-naphthyridin-2(1H)-one,

3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-(4-pyridyl)-1,6-naphthyridin-2(1H)-one, and

3-(3-Cyclopropyl-1,2,4-oxadiazol-5-yl)-5-(3-thienyl)-1,6-naphthyridin-2(1H)-one.

The pharmaceutically acceptable acid addition salt of the compounds of the formula (I) includes inorganic acid addition salts such as hydrochloride, hydrobromide, hydroiodide, sulfate, phosphate, and organic acid addition salts such as oxalate, maleate, fumarate, malonate, lactate, malate, citrate, tartrate, benzoate, methanesulfonate, or tosylate.

In the specification, the terms “lower alkyl group” and the “lower alkyl” moiety mean a straight chain or branched chain alkyl group having 1 to 6 carbon atoms, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, and hexyl groups.

The term “cyclo-lower alkyl group” means a cycloalkyl group having 3 to 6 carbon atoms, for example, cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl groups, where the ring may be substituted by a C₁-C₃ alkyl group or a halogen atom.

The terms “lower alkenyl group” and the “lower alkynyl group” have a straight or branched chain having 2 to 6 carbon atoms, and include, for example, allyl, 1-propenyl, propargyl, and 2-methyl-1-ethynyl groups.

The term “cyclo-lower alkenyl group” means a cycloalkyl group having 5 to 6 carbon atoms, for example, cyclohexenyl group.

The terms “lower alkoxy group” and the “lower alkoxy” moiety mean a straight chain or branched chain alkoxy group having 1 to 6 carbon atoms, and include, for example, methoxy, ethoxy, propoxy, isopropyloxy, butyloxy, isobutyloxy, tert-butyloxy, pentyloxy, and hexyloxy groups.

The terms “aryl group” and “aryl” moiety mean a phenyl group or a naphthyl group and the ring thereof may optionally have a 1 to 3 substituents selected from a halogen atom, a C₁-C₃ alkyl group, a trifluoromethyl group, a hydroxy group, a C₁-C₃ alkoxy group, a trifluoromethoxy group, a cyano group and an amino group, and a nitro group.

The term “heteroaromatic group” means a 5- or 6-membered aromatic heterocyclic group containing, the same or different, 1 to 2 hetero atoms selected from nitrogen atom, oxygen atom, and sulfur atom, and includes, for example, furyl, thienyl, pyrrolyl, oxazolyl, isoxazolyl, pyridyl, pyridazinyl, and pyrimidinyl, which these heteroaromatic groups may optionally have 1 to 3 substituents selected from a halogen atom, a C₁-C₃ alkyl group, a hydroxy group, a C₁-C₃ alkoxy group, and an amino group.

In the “substituted or unsubstituted benzyloxycarbonyl group”, the substituent is selected from a C₁-C₃ alkyl group, a C₁-C₃ alkoxy group, a cyano group and a nitro group.

The term “halogen atom” means fluorine, chlorine, bromine or iodine atom.

The compounds of this invention may be prepared by the processes 1 to 4 as mentioned below.

(Process 1)

In the compound of the formula (Ia):

wherein R¹ and R² are the same as defined above,

or of the formula (Ib):

wherein R¹ and R² are the same as defined above,

when R¹ is a group other than a lower alkoxy group, the compound can be prepared by subjecting a compound of the formula (II):

wherein R¹′ is the same as R¹ other than lower alkoxy group, and

R² is as defined above,

or of the formula (III):

wherein R¹′ is the same as R¹ other than lower alkoxy group, and

R² is as defined above,

to an intramolecular cyclization reaction.

The cyclization reaction may be carried out in the presence of a dehydrating agent, but may usually be carried out by heating the compound in an appropriate solvent which does not affect the reaction. The solvent includes aromatic hydrocarbons (e.g. benzene, toluene, xylene), ethers (e.g. tetrahydrofuran, dioxane), N,N-dimethylformamide. These solvents may be used alone or in combination of two or more thereof. The reaction temperature may vary depending on the kinds of the starting materials, etc. but is usually in the range of 50 to 150° C., preferably 80 to 120° C.

(Process 2)

In the compound of the formula (Ia), when R¹ is a lower alkoxy group, the compounds can be prepared by subjecting a compound of the formula (IV):

wherein R¹″ is a lower alkoxy group, Ph means a phenyl group, and

R² is the same as defined above,

to an intramolecular cyclization reaction in a similar manner as described, for example, in Synthesis, p.843 (1986).

The cyclization reaction is usually carried out by heating the starting compound in an appropriate solvent. The solvent includes aromatic hydrocarbons (e.g. benzene, toluene, xylene), ethers (e.g. tetrahydrofuran, dioxane). The reaction temperature may vary depending on the kinds of the starting materials, etc. but is usually in the range of 50 to 150° C., preferably 80 to 120° C.

(Process 3)

In the compound of the formula (Ib), when R¹ is a lower alkoxy group, the compound can be prepared by reacting a compound of the formula (V):

wherein R¹″ is a lower alkoxy group, and R² is the same as defined above,

with a hydroxylamine in a similar manner as described, for example, in Journal of Heterocyclic Chemistry, vol. 18, p.1197 (1981).

The reaction is usually carried out in an appropriate solvent. The solvent includes alcohols (e.g. methanol, ehtanol), water. The reaction temperature may vary depending on the kinds of the starting materials, etc. but is usually in the range of 50 to 90° C.

(Process 4)

The compound of the formula (Ic):

wherein R¹ and R² are the same as defined above,

can be prepared by subjecting a compound of the formula (VI):

wherein R¹ and R² are the same as defined above,

to an intramolecular cyclization reaction.

The cyclization reaction may be carried out in the presence of a dehydrating agent, but may usually be carried out by heating the compound in an appropriate solvent which does not affect the reaction. The solvent includes aromatic hydrocarbons (e.g. benzene, toluene, xylene), ethers (e.g. tetrahydrofuran, dioxane), N,N-dimethylformamide. These solvents may be used alone or in combination of two or more thereof. The reaction temperature may vary depending on the kinds of the starting materials, etc. but is usually in the range of 50 to 150° C., preferably 80 to 120° C.

The cyclization reaction may also be carried out in a similar manner as described in EP-A2-0588500 in an appropriate solvent which does not affect the reaction in the presence of a trivalent phosphorus compound (e.g. triphenylphosphine) and a dialkylazodicarboxylic acid ester. The reaction temperature may vary depending on the kinds of the starting materials, etc. but is usually in the range of 0 to 110° C., preferably 0 to 60° C.

The compounds (I) of this invention prepared by the above processes 1 to 4 may be isolated and purified by a conventional procedures such as chromatography, recrystallization, or r reprecipitation.

The compounds (I) of this invention may be obtained in the form of a free base or an acid addition salt thereof depending, for example, on the kinds of the selected starting materials to be used, on the reaction conditions and procedures. The acid addition salt may be converted into a free base by treating it by a conventional base such as an alkali metal carbonate and an alkali metal hydroxide. In addition, the free base may be converted into an acid addition salt by treating it with a kind of various acids in a usual manner.

The processes for preparing the starting compounds are explained below.

The compounds of the formulae (II) to (VI) used in the above Processes 1 to 4 are novel compounds and can be prepared by a process as shown in the following Reaction Scheme-1.

wherein R¹′ is the same as R¹ except a lower alkoxy group, and

R² is the same as defined above.

The compound (1) is reacted with hydroxylamine in a usual manner to give the compound (2), and said compound is reacted with a reactive derivative at the carboxyl group of a carboxylic acid of the formula: R¹′COOH (wherein R¹′ is as defined above) in the presence of a base to give the compound of the formula (II).

The compound of the formula (III) used in the above Process 1 may be prepared by a process as shown in the following Reaction Scheme-2.

wherein R¹″ is the same as R¹ except a lower alkoxy group, and

R² is the same as defined above.

The compound (3) or a reactive derivative at the carboxyl group thereof is reacted with a kind of various amidoximes (4) under a reaction condition for a conventional amidation to give the compound of the formula (III).

The compound of the formula (IV) used in the above Process 2 can be prepared by a process as shown in the following Reaction Scheme-3.

wherein R¹″ is a lower alkoxy group, R′ is a lower alkyl group or a substituted or unsubstituted benzyl group, Ph means a phenyl group and R² is the same as defined above.

The compound (5) is reduced with a reducing agent such as sodium borohydride, tetrabutylammonium borohydride, lithium aluminum hydride in an appropriate solvent to give the compound (6) and then said compound is oxidized with an activated manganese dioxide in an appropriate solvent to give the compound (7).

The compound (7) is reacted with hydroxylamine under a condition for the conventional oxime-forming reaction to give the compound (8), and then said compound is reacted with N-chlorosuccinimide in a similar manner as described, for example, in Journal of Organic Chemistry, vol. 45, p.3916 (1980) to give the compound (9).

The compound (9) is reacted with sodium azide in an appropriate solvent in a similar manner as described, for example, in Synthesis, p.102 (1979) to give the compound (10), and then, said compound is reacted with a compound of the formula: XCOR¹″ (wherein X is a halogen atom and R¹″ is a lower alkoxy group) in an appropriate solvent in a similar manner as described, for example, in Synthesis, p.843 (1986) to give the compound (11), and said compound is further reacted with triphenylphosphine to give the compound of the formula (IV).

The compound of the formula (V) used in the above Process 3 can be prepared by a process as shown in the following Reaction Scheme-4.

wherein R¹″ is a lower alkoxy group and R² is the same as defined above.

The compound (3) or a reactive derivative at the carboxyl group thereof is reacted with an alkali metal thiocyanate in an appropriate solvent to give the compound (12) and then said compound is subjected to alcoholysis to give the compound (V).

The compound of the formula (VI) used in the above Process 4 can be prepared by a process as shown in the following Reaction Scheme-5.

wherein R¹ and R² are the same as defined above.

The compound (3) or a reactive derivative at the carboxyl group thereof is reacted with a hydrazide (13) of the formula: R¹CONHNH₂ (wherein R¹ is the same as defined above) by a conventional amidation reaction to give the compound of the formula (IV).

The compound (VI) may also be prepared by a two step reaction, that is, by reacting the compound (3) or a reactive derivative at the carboxyl group thereof with the hydrazine by a conventional amidation reaction, followed by reacting the resultant with a reactive derivative at the carboxyl group of a carboxylic acid of the formula: R¹COOH (R¹ is the same as defined above).

A process for preparing the intermediate of the formula (I′) is explained below.

The compounds of the formula (I′) wherein R is a cyano group or a carboxyl group, that is, the compound (1) and the compound (3) as used in the Reaction Scheme-1 and Reaction Scheme-2, can be prepared in a similar manner as described, for example, in Journal of Heterocyclic Chemistry, vol. 27, p.2085 (1990) or Journal of Medicinal Chemistry, vol. 35, p.4858 (1992) as shown in the following Reaction Scheme-6.

wherein X is a di-lower alkylamino group, a cyclic amino group, a hydroxy group, a halogen atom, or a lower alkoxy group, R′ is a lower alkyl group or a substituted or unsubstituted benzyl group, and R² is the same as defined above.

In the above reaction scheme, the compound (16) can be prepared by reacting the compound (14) with N,N-dimethyl-formamide dimethylacetal or an orthoformic acid ester in an appropriate solvent in a similar manner as described, for example, in Heterocycles, vol. 29, p.1517 (1989) or in Journal of Heterocyclic Chemistry, vol. 27, p.511 (1990) to give the compound (15), followed by reacting it with cyanoacetamide in the presence of an appropriate base.

The compound (16) thus prepared is further reacted with N,N-dimethylformamide dimethylacetal in an appropriate solvent to give the compound (17), and then said compound is reacted with ammonia or an ammonium salt in an approppriate solvent to give the compound (1). The compound (1) thus obtained is hydrolyzed with an acid or an alkali by a conventional method to give the compound (3).

In addition, the compounds of the formula (I′) wherein R is a lower alkoxycarbonyl group or a substituted or unsubstituted benzyloxycarbonyl group, for example, the compounds (5) can be prepared by esterifying the compound (1) or the compound (3) by a conventional method.

PHARMACOLOGICAL EXPERIMENTS

The pharmacological properties of the compounds (I) of the present invention are illustrated by the following experiments with representative compounds.

Experiment 1 Benzodiazepine Receptor Binding Assay

According to the method disclosed in Life Science Vol. 20, p. 2101 (1977), the benzodiazepine receptor binding assay was carried out.

A crude synaptosome membrane fraction prepared from brains of Wistar rats (age: 7 to 8 weeks) was suspended in 15 mM Tris-HCl buffer (pH 7.4) containing 118 mM sodium chloride, 4.8 mM potassium chloride, 1.28 mM calcium chloride and 1.2 mM magnesium sulfate in a concentration of 1 g (wet weight) of brain per 20 ml of buffer to give a receptor membrane source. [3H]-diazepam was used as a labelled ligand.

A test compound (a known amount), [3H]-diazepam (final concentration; 1.5 nM), receptor membrane and the above buffer were added to a test tube (final volume: 1 ml). The reaction was started by addition of the receptor membrane. The test tube was incubated at 0° C. for 20 minutes, and the reaction mixture was terminated by rapid filtration through Whatman GF/B glass fiber filter attached to a Cell-harvester (manufactured by Brandell). Immediately, the collected labelled ligand-bound receptor membrane was washed three times with ice-cold 50 mM Tris-HCl buffer (pH 7.7, each 5 ml). The radioactivity on the filter was measured by a liquid scintillation counter to determine the amount of the [³H]-diazepam bound to the receptor membrane (total binding). Separately, the same procedures were repeated except 1 μM diazepam was added, and thereby the amount of [³H]-diazepam bound to the receptor membrane (non-specific binding) was measured likewise. This non-specific binding was deducted from the total binding to give the specific binding. Based on the specific binding thus obtained, the inhibitory activity (IC50) of the test compound was determined by probit method.

The results are shown in the following Tables 1 to 4.

TABLE 1 BENZODIAZEPINE RECEPTOR BINDING Assay BZP-receptor BZP-receptor Binding Binding Ex. No. IC₅₀ (nM) Ex. No. IC₅₀ (nM) 1 2.28 26 0.67 3 3.58 27 0.96 4 1.65 28 1.25 5 1.64 29 5.28 6 2.98 31 1.64 7 2.39 32 3.29 9 1.62 33 5.50 10 8.08 37 3.91 11 9.77 38 1.31 12 7.89 39 2.86 14 9.45 40 7.45 15 6.16 44 2.62 16 3.69 45 0.96 17 0.69 46 2.15 18 2.04 47 2.33 19 6.37 48 1.49 20 2.77 49 1.11 21 4.21 50 0.88 22 3.76 51 0.79 23 1.76 52 0.74 24 4.47 54 1.21 25 1.84 55 1.66

TABLE 2 BZP-receptor BZP-receptor Binding Binding Ex. No. IC₅₀ (nM) Ex. No. IC₅₀ (nM) 56 2.71 104 5.18 57 1.55 105 1.08 58 1.52 106 1.96 59 1.98 107 6.56 60 2.01 108 2.14 61 1.04 109 1.75 86 2.21 110 1.16 87 2.35 111 2.06 88 4.63 112 2.68 89 10.5 113 2.18 91 0.61 114 1.08 92 0.75 115 1.52 93 1.75 116 1.17 94 4.49 117 1.41 95 1.09 118 1.28 96 2.82 119 2.53 97 4.64 120 1.59 98 8.56 121 0.78 99 1.67 122 0.87 100 1.31 123 1.12 102 0.81 173 0.94 103 0.83 175 1.21

TABLE 3 BZP-receptor BZP-receptor Binding Binding Ex. No. IC₅₀ (nM) Ex. No. IC₅₀ (nM) 176 2.13 203 1.97 176 2.33 204 5.51 178 2.31 205 4.77 179 4.73 206 1.16 180 1.22 207 3.42 183 1.55 208 4.14 186 1.55 209 1.28 187 6.42 210 3.41 188 1.20 211 0.82 189 0.84 212 1.26 190 1.57 213 2.07 192 4.22 215 2.47 193 4.10 216 1.17 194 1.06 217 1.34 195 4.01 218 2.58 196 4.60 219 2.03 197 1.97 220 0.93 198 1.03 221 0.72 199 1.55 222 1.49 200 0.92 224 3.57 201 1.84 225 2.12 202 2.09 226 1.41

TABLE 4 BZP-receptor Binding Ex. No. IC₅₀ (nM) 227 1.46 228 1.59 229 1.12 230 0.9 231 0.71 232 6.48 233 1.58 234 0.84 235 0.91 236 1.61 237 1.86 238 1.38 240 2.51 241 6.08 242 1.87 243 1.81 244 4.12 245 0.81 246 1.46 247 1.39 311 1.91

Experiment-2 TBPS Binding Assay

(Method)

The TBPS (t-butylbicyclophosphonothionate) binding assay and the preparation of the membrane specimen were done in a similar manner to the method of Biggio, G. et al. [cf. European Journal of Pharmacology, vol. 161, pp.173-180 (1989)].

The membrane specimen was prepared from the cerebral cortex of Wistar rats (age: 7 to 8 weeks) by the following procedure. That is, to the cerebral cortex was added a 50-fold volume of an ice-cooled buffer (a 50 mM Tris-citrate buffer containing 100 mM sodium chloride, pH 7.4) and the mixture was homogenized at 0-4° C. and then centrifuged at 20,000 g for 20 minutes. The pellets thus obtained were once subjected to homogenization in a buffer and centrifugation by the same procedure as above and then kept in freezed state at −80° C. for more than 20 hours. On the test day, the freezed pellets were thawed and then subjected twice to the homogenization-centrifugation procedure as described above. The pellets thus obtained were suspended in a buffer in a concentration of 1 g (wet weight) per 25 ml of buffer to give a membrane specimen to be used in the binding assay.

The binding assay was carried out by the following procedure by using as a labelled ligand [³⁵S]TBPS (final concentration; 0.4 nM) and as a non-labelled ligand Picrotoxin (final concentration; 100 μM) in the presence of GABA (final concentration; 1 μM).

A test compound (a known amount), [³⁵S] labelled ligand, the membrane specimen, GABA and a buffer were added to a test tube (final volume; 1 ml). The reaction was started by addition of the membrane specimen (200 μl). The test tube was incubated at 25° C. for 90 minutes, and the reaction was terminated by filtration through Wattman GF/B glass fiber filter (which was previously dipped in 0.01% polyethylenimine for one day) attached to a Cell-harvester (manufactured by Brandell), and thereby, the labelled ligand-bound membrane was collected onto the filter. Immediately, the collected labelled ligand-bound membrane was washed with a ice-cooled 50 mM Tris-HCl buffer (pH 7.7, each 5 ml) three times. Subsequently, the filter was moved into a liquid scintillation vial and thereto was added a liquid scintillation cocktail (ACS-II, manufactured by Amersham, USA, 10 ml) and allowed to stand for a fixed period of time. Thereafter, the radioactivity on the filter was measured by a liquid scintillation counter (2000CA type, manufactured by Paccard, USA) to determine the total binding amount. Separately, the same procedures were repeated in the presence of Picrotoxin to determine the non-specific binding amount. The non-specific binding amount was deducted from the total binding amount to give the specific binding amount. The binding activity of the test compound was calculated by a variation rate, i.e., a rate of the specific binding amount of the test compound to the specific binding amount in control (using a solvent).

(Evaluation criteria)

+% value means to exhibit inverse agonistic properties, −% value means to exhibit agonistic properties and 0% means to exhibit antagonistic properties.

The results are shown in Table 5 and Table 6.

TABLE 5 TBPS Binding Test Variation Variation Ex. No. Rate (%) Ex. No. Rate (%) 2 53 48 18 3 12 50 40 4 20 52 −11 5 21 55 −16 6 15 58 −15 7 12 86 12 8 13 87 32 11 21 88 41 13 24 89 22 14 38 91 38 16 38 92 32 17 25 93 34 18 10 94 34 20 −13 95 13 21 21 96 16 23 −23 100 −23 24 22 102 33 36 15 103 21 38 25 105 25 43 28 106 20 45 −9 107 17 47 −11 123 12

TABLE 6 Variation Variation Ex. No. Rate (%) Ex. No. Rate (%) 173 18 194 32 175 28 196 −9 176 39 198 11 177 29 200 17 178 25 202 22 179 37 203 17 180 13 206 −13 181 13 210 15 182 19 211 24 183 8 212 27 184 17 213 26 185 19 224 −12 186 21 228 −10 187 27 229 10 188 34 233 31 189 29 240 −13 190 34 241 −14 191 18 244 −15 192 13 245 13 193 44 247 16

Experiment 3 Test of Increasing Activity on Pentylentetrazol-induced Convulsion

It is known that benzodiazepine receptor inverse agonists increase convulsion induced by pentylentetrazol [cf. Progress in Neuro-Psychopharmacology and Biological Psychiatry, vol. 12, p.951 (1988)]. Some compounds of the present invention were tested as to the activities of increasing the pentylentetrazol-induced convulsion.

A test compound (compounds disclosed in working examples) was orally administered to ddY male mice (weight; 22-25 g, five mice/group) in an amount of 5-100 mg/kg. Fifteen minutes later, pentylentetrazol (70 mg/kg, which amount does not induce tonic convulsion by said compound alone) was injected subcutaneously into the mice, and immediately, the mice were observed as to the appearance of tonic convulsion at the hind leg for 30 minutes. The effects were evaluated by the number of mice among five mice, of which the convulsion increasing effects were observed. The results are shown in Table 7.

TABLE 7 Effects Dose (number of Ex. No. (mg/kg, po) animals) 4 10 4/5 5 5 5/5 6 100 4/5 11 10 4/5 18 20 3/5 86 10 2/5 88 10 5/5 89 50 5/5 94 50 4/5 102 10 5/5 103 10 2/5 105 50 4/5 173 20 5/5 183 20 4/5 184 50 5/5 186 50 4/5 191 10 5/5 192 100 5/5 200 50 4/5 210 10 4/5 213 10 5/5 220 20 4/5 247 50 4/5

As is shown in the above results, compounds embodying the present invention showed high selective affinity for benzodiazepine receptor and hence are useful as a drug for acting onto benzodiazepine receptor. Although some of the compounds of this invention have also agonistic properties, the compounds of this invention are particularly useful as an inverse agonist. The compounds having inverse agonistic properties are expected to be used in clinical fields entirely different from those of agonists, for example, as a psychoanaleptic drug or a drug for the treatment of dysmnesia in senile dementia or Alzheimer's disease.

PHARMACEUTICAL USE OF THE COMPOUNDS OF THIS INVENTION

The compounds of this invention may be administered either orally, parentally or intrarectally when used as a drug for acting onto benzodiazepinereceptor,butpreferablyorally. The dosage of the compounds varies according to the route of the administration, conditions and ages of the patients, or the types of the treatment (e.g. prophylaxis or treatment) and the like, but it is usually in the range of 0.01 to 10 mg/kg/day, preferably in the range of 0.02 to 5 mg/kg/day.

The present compounds may be administered in the form of a conventional pharmaceutical preparation in admixture with a conventional pharmaceutically acceptable carrier or diluent. The pharmaceutically acceptable carrier or diluent may be any conventional one which is used in this field and does not react with the present compound, for example, lactose, glucose, mannitol, dextran, starch, white sugar, magnesium aluminate metasilicate, synthetic aluminum silicate, crystalline cellulose, sodium carboxymethylcellulose, calcium carboxymethylcellulose, hydroxypropyl starch, ion-exchange resin, methyl cellulose, gelatin, gum arabic, hydroxypropyl cellulose, lower-substituted hydroxypropyl cellulose, hydroxypropylmethyl cellulose, polyvinylpyrrolidone, polyvinyl alcohol, light silicic anhydride, magnesium stearate, talc, carboxyvinyl polymer, titanium oxide, sorbitan fatty acid ester, sodium laurylsulfate, glycerin, glycerin fatty acid ester, purified lanolin, glycerogelatin, polysorbate, macrogol, vegetable oil, wax, liquid paraffin, white petrolatum, nonionic surfactant, propylene glycol, water, and the like.

Pharmaceutical preparations are tablets, capsules, granules, powders, syrups, suspensions, suppositories, gels, injection preparations, and the like. These preparations may be prepared by a conventional method. When a liquid preparation is prepared, it may previously be in the form of a solid preparation which is dissolved or suspended in water or a solvent when used. In addition, tablets or granules may be coated by a conventional method, and injection preparations prepared by dissolving the compound (I) of the present invention or an acid addition salt thereof in distilled water for injection, or a physiological saline solution, but if necessary, it may be dissolved in a isotonic solution, and further, a pH adjustor, a buffer or a preservative may be added thereto.

These pharmaceutical preparations may contain the present compound in an amount of more than 0.01 % by weight, preferably 0.05 to 70 % by weight, and may contain other pharmacologically active ingredients.

BEST MODE FOR CARRYING OUT THE INVENTION

The compounds of this invention are illustrated by the following Examples. The symbols in tables means as follows. Me: methyl, Et: ethyl, n-Pr: n-propyl, i-Pr: isopropyl, c-Pr: cyclopropyl, n-Bu: n-butyl, t-Bu: tert-butyl, Ph: phenyl. The position of substituents is indicated like this, for example, 3-Me-Ph means 3-methylphenyl.

EXAMPLE 1 Preparation of 3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-5-methyl-1,6-naphthyridin-2(1H)-one

(1) To a solution of hydroxylamine hydrochloride (4.17 g) in water (50 ml) was added sodium carbonate (3.18 g) with stirring under ice cooling. To the solution were subsequently added ethanol (200 ml) and 1,2-dihydro-5-methyl-2-oxo-1,6-naphthyridine-3-carbonitrile (3.70 g), and the mixture was refluxed for 2 hours. After distilling off the solvent under reduced pressure, water was added to the residue, and the precipitated crystals were separated by filtration. The product was washed with water, isopropanol, diisopropyl ether in this order, and dried to give 1,2-dihydro-5-methyl-2-oxo-1,6-naphthyridin-3-amidoxime (4.2 g). This compound was used in the next reaction without being purified.

(2) To a suspension of the above amidoxime (1.09 g), sodium carbonate (0.83 g) and methyl ethyl ketone (200 ml) was added cyclopropanecarbonyl chloride (0.57 g) with stirring under ice cooling, and the mixture was stirred at room temperature overnight. After distilling off the solvent under reduced pressure, water was added to the residue, and the precipitated crystals were separated by filtration, washed with water, isopropanol and diisopropyl ether in this order, and then dried. To the resulting crystals was added dimethylformamide (DMF) (50 ml) and the mixture was stirred at 130° C. for 5 hours. After distilling off the solvent under reduced pressure, isopropanol was added to the residue, and the crystals were separated by filtration. The resulting crystals were recrystallized from ethanol-chloroform to give the title compound (0.65 g) as colorless crystals. M.p. 259-260° C.

EXAMPLES 2 to 85

In the same manner as described in Example 1, the corresponding starting materials were reacted to give the compounds of Examples 2 to 85 as shown in Tables 8 to 12.

TABLE 8

Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation  2 Me Me >300 CHCl₃—EtOH  3 Me i-Bu 231-233 MeCN  4 Me Ph 302-303 EtOH  5 Me 3-F—Ph >300 EtOH  6 Me 4-F—Ph >300 EtOH  7 Me 3-Cl—Ph >300 CHCl₃—MeOH  8 Me 4-Cl—Ph >300 CHCl₃—MeOH  9 Me 3-Me—Ph 285-287 EtOH 10 Me 4-Me—Ph >300 MeCN 11 Me 4-MeO—Ph 298-300 CHCl₃—MeCN 12 Et H 249-251 MeCN 13 Et Me 246-247 CHCl₃—EtOH 14 Et Et 239-240 EtOH 15 Et n-Pr 222-223 MeCN 16 Et i-Pr 288-289 MeCN 17 Et c-Pr 266-268 MeCN 18 Et 2-Me-c-Pr 285-287 EtOH 19 Et n-Bu 223-225 MeCN 20 Et i-Bu 227-229 MeCN

TABLE 9 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 21 Et c-Bu 271-272 EtOH 22 Et n-pentyl 216-218 MeCN 23 Et c-hexyl 295-296 CHCl₃—MeCN 24 Et c-hexyl-CH₂ 235-237 EtOH 25 Et 3-c-hexenyl 281-282 EtOH 26 Et Ph 289-290 EtOH 27 Et 3-F—Ph >300 EtOH 28 Et 4-F—Ph >300 EtOH 29 Et 3-Cl—Ph >300 CHCl₃—MeOH 30 Et 4-Cl—Ph >360 EtOH 31 Et 3-Me—Ph 290-291 CHCl₃—EtOH 32 Et 4-Me—Ph >300 EtOH 33 Et 4-MeO—Ph >300 CHCl₃—EtOH 34 n-Pr Me 252-254 EtOH 35 n-Pr Et 216-218 MeCN 36 n-Pr i-Pr 273-274 MeCN 37 n-Pr c-Pr 234-236 MeCN 38 n-Pr Ph 284-285 EtOH 39 n-Pr 3-F—Ph >300 EtOH 40 n-Pr 4-F—Ph >300 EtOH 41 i-Pr Me 266-267 EtOH

TABLE 10 Solvent for Ex. No. R¹ R² M.p. (° C.) recrystallization 42 i-Pr Et 247-249 MeCN 43 i-Pr i-Pr 265-267 MeCN 44 i-Pr c-Pr 276-278 EtOH 45 i-Pr Ph 280-281 EtOH 46 i-Pr 3-F—Ph >300 EtOH 47 i-Pr 4-F—Ph >300 EtOH 48 c-Pr Et 233-236 MeCN 49 c-Pr n-Pr 217-219 MeCN 50 c-Pr i-Pr 268-269 MeCN 51 c-Pr c-Pr 242-245 MeCN 52 c-Pr Ph 281-282 EtOH 53 c-Pr 3-F—Ph >300 EtOH 54 c-Pr 4-F—Ph >300 EtOH 55 c-Pr 3-Cl—Ph >300 EtOH 56 c-Pr 4-Cl—Ph >300 EtOH 57 c-Pr 3-Me—Ph 274-276 EtOH 58 c-Pr 4-Me—Ph >300 EtOH 59 c-Pr 4-MeO—Ph 297-299 EtOH 60 i-propenyl c-Pr 265-268 MeCN 61 vinyl c-Pr 250-252 MeCN

TABLE 11 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 62 i-Pr 2-Cl—Ph 63 i-Pr 3-Cl—Ph 283-285 EtOH 64 i-Pr 4-Cl—Ph >300 EtOH 65 i-Pr 2-Br—Ph 66 i-Pr 3-Br—Ph >300 CHCl₃—EtOH 67 i-Pr 4-Br—Ph >300 CHCl₃—EtOH 68 i-Pr 2-Me—Ph >300 MeCN 69 i-Pr 3-Me—Ph 281-283 MeCN 70 i-Pr 4-Me—Ph 293-294 EtOH 71 i-Pr 3-MeO—Ph 202-204 MeCN 72 i-Pr 3-CF₃—Ph 258-261 EtOH 73 i-Pr n-Pr 217-219 MeCN 74 n-Bu Ph 264-266 MeCN 75 i-Bu Ph 281-283 MeCN 76 t-Bu Ph 294-296 MeCN 77 c-hexyl Ph 276-277 MeCN 78 CH₂OCH₃ Ph 79 Ph Ph >300 DMF 80 2-Cl—Ph Ph >300 CHCl₃—EtOH 81 3-Cl—Ph Ph >300 DMF 82 4-Cl—Ph Ph >300 DMF

TABLE 12 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 83 4-Me—Ph Ph >300 DMF 84 3-pyridyl Ph >300 CHCl₃—EtOH 85 2-furyl Ph >300 EtOH

EXAMPLE 86 Preparation of 3-(5-methyl-1,2,4-oxadiazol-3-yl)-5-(3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one

To a solution of acetic acid (0.90 g) in DMF (100 ml) was added N,N′-carbonyldiimidazole (2.43 g) and the mixture was stirred at 70° C. for 3 hours. To the solution was added 1,2-dihydro-5-(3-methoxyphenyl)-2-oxo-1,6-naphthyridin-3-amidoxime (3.10 g) prepared in the same manner as described in Example 1(1), and the mixture was stirred at 70° C. for 2 hours and further at 130° C. for 1 hour. The reaction mixture was concentrated to dryness under reduced pressure, and water was added to the residue, and the precipitated crystals were separated by filtration and washed with water, isopropanol and diisopropyl ether in this order and then dried. The resulting crystals were subjected to silica gel column chromatography and eluted with chloroform-methanol (50:1). The resulting crystals were recrystallized from chloroform-ethanol to give the title compound (2.22 g) as colorless crystals. M.p. 286-288° C. Hydrochloride of the title compound, M.p. 281-282° C. (recrystallized from ethanol).

EXAMPLES 87 to 172

In the same manner as described in Example 86, the corresponding starting materials were reacted to give the compounds of Examples 87 to 172 as shown in Tables 13 to 17.

TABLE 13

Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 87 Me 2-F—Ph 287-288 EtOH-i-Pr₂O 88 Me 2-Cl—Ph 262-264 CHCl₃—MeOH 89 Me 2-Me—Ph 244-245 MeCN 90 Me 2-MeO—Ph 260-262 MeCN 91 Me 2-furyl 291-294 MeOH 92 Me 2-thienyl >300 CHCl₃—MeOH 93 Me 3-thienyl 292-294 MeCN 94 Me 4-pyridyl >300 EtOH-i-Pr₂O 95 Et 2-F—Ph 235-236 EtOH-i-Pr₂O 96 Et 2-Cl—Ph 167-168 EtOH 97 Et 2-Me—Ph 221-222 EtOH 98 Et 2-MeO—Ph 229-230 EtOH—I—Pr₂O 99 Et 3-MeO—Ph 246-247 EtOH 100  Et 1-naphthyl 248-250 MeCN

TABLE 14 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 101 Et 2-napthyl 231-233 MeCN 102 Et 2-furyl 278-280 CHCl₃—MeCN 103 Et 2-thienyl 291-294 CHCl₃—MeCN 104 Et 5-Cl-2- 280-281 CHCl₃—EtOH thienyl 105 Et 3-thienyl >300 EtOH 106 Et 3-pyridyl 292-293 EtOH 107 Et 4-pyridyl >300 CHCl₃—MeOH 108 n-Pr 2-F—Ph 242-243 MeCN 109 n-Pr 2-furyl 264-266 EtOH 110 n-Pr 2-thienyl 269-271 EtOH 111 n-Pr 3-thienyl 296-297 EtOH 112 i-Pr 2-F—Ph 265-267 MeCN 113 i-Pr 2-furyl 274-276 EtOH 114 i-Pr 2-thienyl 273-275 MeCN 115 i-Pr 3-thienyl 297-299 EtOH 116 c-Pr 2-F—Ph 133-135 MeCN 117 c-Pr 2-Cl—Ph 270-272 MeCN 118 c-Pr 2-Me—Ph 265-267 MeCN 119 c-Pr 2-MeO—Ph 175-177 MeCN 120 c-Pr 3-MeO—Ph 146-148 EtOH 121 c-Pr 2-furyl 280-282 CHCl₃—EtOH 122 c-Pr 2-thienyl >300 CHCl₃—EtOH 123 c-Pr 3-thienyl 289-290 EtOH

TABLE 15 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 124 n-Pr 2-Cl—Ph 213-215 MeCN 125 n-Pr 3-Cl—Ph 298-300 EtOH 126 n-Pr 4-Cl—Ph >300 EtOH 127 Me 2-Br—Ph 270-273 MeCN 128 Et 2-Br—Ph 149-151 MeCN 129 c-Pr 2-Br—Ph 260-262 MeCN 130 n-Pr 2-Br—Ph 293-296 MeCN 131 Me 3-Br—Ph >300 CHCl₃—EtOH 132 Et 3-Br—Ph >300 EtOH 133 c-Pr 3-Br—Ph >300 CHCl₃—EtOH 134 n-Pr 3-Br—Ph >300 CHCl₃—EtOH 135 Me 4-Br—Ph >300 CHCl₃—EtOH 136 Et 4-Br—Ph >300 EtOH 137 c-Pr 4-Br—Ph >300 CHCl₃—EtOH 138 n-Pr 4-Br—Ph >300 CHCl₃—EtOH 139 n-Pr 2-Me—Ph 208-210 MeCN 140 n-Pr 3-Me—Ph 226-228 MeCN 141 n-Pr 4-Me—Ph 253-255 MeCN 142 Me 2-OH—Ph 143 Et 2-OH—Ph 144 Me 3-OH—Ph 145 Et 3-OH—Ph 146 Me 4-OH—Ph

TABLE 16 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 147 Et 4-OH—Ph 148 n-Pr 2-MeO—Ph 193-194 MeCN 149 n-Pr 3-MeO—Ph 239-241 MeCN 150 Me 3-CF₃—Ph >300 CHCl₃—EtOH 151 Et 3-CF₃—Ph >300 EtOH 152 c-Pr 3-CF₃—Ph >300 EtOH 153 n-Pr 3-CF₃—Ph 286-288 EtOH 154 Me 4-CF₃—Ph >300 CHCl₃—EtOH 155 Et 4-CF₃—Ph >300 EtOH 156 c-Pr 4-CF₃—Ph >300 CHCl₃—EtOH 157 Me 3-CF₃O—Ph >300 EtOH 158 Et 3-CF₃O—Ph 271-273 MeCN 159 c-Pr 3-CF₃O—Ph 237-239 MeCN 160 Me 4-CF₃O—Ph 161 Et 4-CF₃O—Ph >300 EtOH 162 c-Pr 4-CF₃O—Ph >300 EtOH 163 Me n-Pr 269-271 MeCN 164 Me i-Pr 292-294 MeCN 165 Me c-Pr >300 MeCN 166 Me n-Bu 232-233 MeCN

TABLE 17 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 167 n-Pr n-Bu 220-222 MeCN 168 Me c-hexyl 283-286 MeCN 169 CF₃ Ph 170 CH₂OH Ph 171 2-thienyl Ph >300 CHCl₃—EtOH 172 3-furyl Ph >300 EtOH

EXAMPLE 173 Preparation of 3-(3-ethyl-1,2,4-oxadiazol-5-yl)-5-(2-thienyl)-1,6-naphthyridin-2(1H)-one

To a solution of 1,2-dihydro-5-(2-thienyl)-2-oxo-1,6-naphthyridin-3-carboxylic acid (3.81 g) in DMF (50 ml) was added N,N′-carbonyldiimidazole (3.41 g) and the mixture was stirred at 70° C. for 4 hours. To the solution was added propionic amidoxime (1.85 g), and the mixture was stirred at 70° C. for 1 hour and further at 130° C. for 2 hours. The reaction mixture was concentrated to dryness under reduced pressure, and water was added to the residue, and the precipitated crystals were separated by filtration and washed with water, isopropanol and diisopropyl ether in this order and then dried. The resulting crystals were subjected to silica gel column chromatography and eluted with chloroform-methanol (50:1). The resulting crystals were recrystallized from chloroform-ethanol to give the title compound (2.60 g) as colorless crystals. M.p. 265-268° C.

EXAMPLES 174 to 307

In the same manner as described in Example 173, the corresponding starting materials were reacted to give the compounds of Examples 174 to 307 as shown in Tables 18 to 24.

TABLE 18

Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 174 Me Me >300 EtOH 175 Me Ph >300 EtOH 176 Me 2-F—Ph >300 EtOH 177 Me 3-F—Ph >300 EtOH 178 Me 4-F—Ph >300 EtOH 179 Me 2-Cl—Ph 263-265 MeCN 180 Me 3-Cl—Ph >300 CHCl₃—EtOH 181 Me 4-Cl—Ph >300 CHCl₃—EtOH 182 Me 2-Me—Ph 266-267 MeCN 183 Me 3-Me—Ph 286-287 EtOH 184 Me 4-Me—Ph >300 EtOH 185 Me 2-MeO—Ph >300 MeCN 186 Me 3-MeO—Ph 276-278 EtOH 187 Me 4-MeO—Ph 287-289 EtOH 188 Me 2-furyl >300 EtOH 189 Me 2-thienyl >300 EtOH 190 Me 3-thienyl 294-295 MeCN 191 Et Me 259-260 EtOH

TABLE 19 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 192 Et n-Pr 202-204 MeCN 193 Et i-Pr 222-224 MeCN 194 Et c-Pr 255-257 MeCN 195 Et n-Bu 196-198 MeCN 196 Et i-Bu 198-200 MeCN 197 Et c-hexyl 254-257 MeCN 198 Et Ph 276-277 EtOH 199 Et 2-F—Ph 242-243 EtOH-i-Pr₂O 200 Et 3-F—Ph >300 EtOH 201 Et 4-F—Ph >300 EtOH 202 Et 2-Cl—Ph 260-261 MeCN 203 Et 3-Cl—Ph >300 CHCl₃—MeOH 204 Et 4-Cl—Ph >300 CHCl₃—EtOH 205 Et 2-Me—Ph 245-246 MeCN 206 Et 3-Me—Ph 270-272 MeCN 207 Et 4-Me—Ph 267-269 EtOH 208 Et 2-MeO—Ph 225-226 MeCN 209 Et 3-MeO—Ph 250-252 MeCN 210 Et 4-MeO—Ph 266-268 EtOH 211 Et 2-furyl 254-256 EtOH 212 Et 3-thienyl 286-288 EtOH

TABLE 20 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 213 Et 4-pyridyl >300 MeCN 214 n-Pr Me 217-218 EtOH 215 n-Pr c-Pr 201-202 MeCN 216 n-Pr Ph 224-225 EtOH 217 n-Pr 2-F—Ph 220-221 EtOH 218 n-Pr 3-F—Ph 259-261 EtOH 219 n-Pr 4-F—Ph 261-263 EtOH 220 n-Pr 2-furyl 221-223 EtOH 221 n-Pr 2-thienyl 225-227 EtOH 222 n-Pr 3-thienyl 244-246 EtOH 223 i-Pr Me 244-245 EtOH 224 i-Pr c-Pr 275-277 MeCN 225 i-Pr Ph 273-275 MeCN 226 i-Pr 2-F—Ph 257-259 MeCN 227 i-Pr 3-F—Ph 297-299 EtOH 228 i-Pr 4-F—Ph 284-286 EtOH 229 i-Pr 2-furyl 258-260 EtOH 230 i-Pr 2-thienyl 260-262 EtOH 231 i-Pr 3-thienyl 263-264 EtOH 232 c-Pr Me 251-253 MeCN

TABLE 21 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 233 c-Pr i-Pr 224-227 MeCN 234 c-Pr c-Pr 241-243 MeCN 235 c-Pr Ph 278-280 EtOH 236 c-Pr 2-F—Ph 219-221 MeCN 237 c-Pr 3-F—Ph 289-291 EtOH 238 c-Pr 4-F—Ph 298-300 EtOH 239 c-Pr 2-Cl—Ph 262-263 MeCN 240 c-Pr 3-Cl—Ph >300 EtOH 241 c-Pr 4-Cl—Ph >300 EtOH 242 c-Pr 2-Me—Ph 239-240 MeCN 243 c-Pr 3-Me—Ph 246-248 EtOH 244 c-Pr 4-Me—Ph >300 EtOH 245 c-Pr 2-furyl 277-278 EtOH 246 c-Pr 2-thienyl 281-282 EtOH 247 c-Pr 3-thienyl 280-281 EtOH 248 n-Pr 3-Cl—Ph 293-295 MeCN 249 i-Pr 3-Cl—Ph >300 EtOH 250 n-Pr 4-Cl—Ph 287-289 MeCN 251 i-Pr 4-Cl—Ph >300 EtOH 252 Me 2-Br—Ph 275-278 MeCN 253 Et 2-Br—Ph 254-255 MeCN

TABLE 22 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 254 c-Pr 2-Br—Ph 259-261 MeCN 255 n-Pr 2-Br—Ph 226-229 MeCN 256 i-Pr 2-Br—Ph 238-39  MeCN 257 Me 3-Br—Ph >300 CHCl₃-EtOH 258 Et 3-Br—Ph >300 EtOH 259 c-Pr 3-Br—Ph >300 EtOH 260 n-Pr 3-Br—Ph >300 EtOH 261 i-Pr 3-Br—Ph >300 EtOH 262 Me 4-Br—Ph >300 CHCl₃-EtOH 263 Et 4-Br—Ph >300 MeCN 264 c-Pr 4-Br—Ph >300 EtOH 265 n-Pr 4-Br—Ph >300 EtOH 266 i-Pr 4-Br—Ph >300 EtOH 267 n-Pr 2-Me—Ph 228-230 MeCN 268 i-Pr 2-Me—Ph 223-226 MeCN 269 n-Pr 3-Me—Ph 220-221 MeCN 270 i-Pr 3-Me—Ph 247-249 MeCN 271 n-Pr 4-Me—Ph 252-253 MeCN 272 i-Pr 4-Me—Ph >300 MeCN 273 Me 2-OH—Ph

TABLE 23 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 274 Et 2-OH—Ph 275 Me 3-OH—Ph 276 Et 3-OH—Ph 277 Me 4-OH—Ph 278 Et 4-OH—Ph 279 n-Pr 2-MeO—Ph 220-222 MeCN 280 i-Pr 2-MeO—Ph 267-269 MeCN 281 n-Pr 3-MeO—Ph 219-220 MeCN 282 i-Pr 3-MeO—Ph 225-226 MeCN 283 n-Pr 4-MeO—Ph 220-222 MeCN 284 i-Pr 4-MeO—Ph 272-274 MeCN 285 Me 3-CF₃—Ph >300 EtOH 286 Et 3-CF₃—Ph >300 EtOH 287 c-Pr 3-CF₃—Ph 288-290 EtOH 288 Me 4-CF₃—Ph >300 EtOH 289 Me 3-CF₃O—Ph >300 EtOH 290 Et 3-CF₃O—Ph 293-295 MeCN 291 c-Pr 3-CF₃O—Ph 252-254 MeCN 292 Et 4-CF₃O—Ph 279-281 EtOH 293 c-Pr 4-CF₃O—Ph 279-281 MeCN

TABLE 24 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 294 Me n-Pr 242-244 MeCN 295 c-Pr n-Pr 204-205 MeCW 296 Me c-Pr >300 EtOH 297 Ph Ph >300 CHCl₃—EtOH 298 2-Cl—Ph Ph >300 CHCl₃—EtOH 299 3-Cl—Ph Ph 263-265 DMF 300 4-Cl—Ph Ph >300 DMF 301 2-Me—Ph Ph >300 EtOH 302 3-Me—Ph Ph 266-267 CHCl₃—EtOH 303 4-Me—Ph Ph 286-287 CHCl₃—EtOH 304 2-pyridyl Ph >300 EtOH 305 3-pyridyl Ph >300 EtOH 306 4-pyridyl Ph >300 EtOH 307 2-thienyl Ph >300 EtOH

EXAMPLE 308 Preparation of 3-(5-ethyl-1,3,4-oxadiazol-2-yl)-5-(2-thienyl-1,6-naphthyridin-2(1H)-one

(1) solution of 1,2-dihydro-5-(2-thienyl)-2-oxo-1,6-naphthyridine-3-carboxylic acid (1.36 g) and N,N′-carbonyl-dimdazole (1.22 g) in DMF (50 ml) was stirred at 70° C. for 4 hours. To the solution was added propionylhydrazide (0.53 g), and the mixture was stirred at 70° C. for 2 hours. The reaction mixture was concentrated to dryness under reduced pressure, and isopropanol was added to the residue, and the precipitated crystals were separated by filtration. The product was washed with isopropanol and diisopropyl ether in this order, and dried to give 1,2-dihydro-N′-propionyl-5-(2-thienyl)-2-oxo-1,6-naphthyridin-3-carbohydrazide (1.21 g) as yellow crystals. This compound was used in the next reaction without being purified.

(2) To a suspension of the above carbohydrazide (1.09 g), triphenylphosphine (1.57 g) and triethylamine (1.06 g) in tetrahydrofuran (THF) (50 ml) was added dropwise diethyl azodicarboxylate (1.04 g) under ice cooling. The mixture was stirred at 70° C. for 4 hours. After cooling, water was added to the mixture, and then the mixture was concentrated under reduced pressure, and isopropanol was added to the residue. The precipitated crystals were separated by filtration and dried. The resulting crystals were subjected to silica gel column chromatography and eluted with chloroform-methanol (50:1). The crystals were recrystallized from ethanol to give the title compound (0.21 g) as colorless crystals. M.p. >300° C.

EXAMPLES 309 to 368

In the same manner as described in Example 308, the corresponding starting materials were reacted to give the compounds of Examples 309 to 368 as shown in Tables 25 to 27.

TABLE 25

Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 309 Me Me >300 MeCN 310 Et Me 258-259 EtOH 311 Et Ph 258-259 MeCN 312 n-Pr Me 209-210 MeCN 313 i-Pr Me 250-251 EtOH 314 c-Pr Me >300 EtOH 315 Me 3-F—Ph >300 CHCl₃—EtOH 316 Et 3-F—Ph >300 EtOH 317 c-Pr 3-F—Ph 288-290 EtOH 318 n-Pr 3-F—Ph 277-279 EtOH 319 i-Pr 3-F—Ph 241-243 MeCN 320 Me 3-Br—Ph 246-248 EtOH 321 Et 3-Br—Ph 292-293 CHCl₃—EtOH 322 c-Pr 3-Br—Ph >300 MeCN 323 Et 4-Br—Ph >300 EtOH 324 c-Pr 4-Br—Ph >300 CHCl₃—EtOH 325 c-Pr 2-Me—Ph 283-285 MeCN 326 Me 3-Me—Ph >300 EtOH 327 Et 3-Me—Ph 258-260 MeCN

TABLE 26 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 328 c-Pr 3-Me—Ph 244-246 MeCN 329 n-Pr 3-Me—Ph 233-235 MeCN 330 i-Pr 3-Me—Ph 187-189 MeCN 331 Me 4-Me—Ph >300 CHCl₃—EtOH 332 Et 4-Me—Ph >300 EtOH 333 c-Pr 4-Me—Ph >300 EtOH 334 n-Pr 4-Me—Ph >300 EtOH 335 i-Pr 4-Me—Ph >300 EtOH 336 Me 2-OH—Ph 337 Et 2-OH—Ph 338 Me 3-OH—Ph 339 Et 3-OH—Ph 340 Me 4-OH—Ph 341 Et 4-OH—Ph 342 Et 2-MeO—Ph 267-269 MeCN 343 Me 3-MeO—Ph >300 EtOH 344 Et 3-MeO—Ph 272-273 MeCN 345 c-Pr 3-MeO—Ph 294-296 MeCN 346 n-Pr 3-MeO—Ph 199-201 MeCN 347 i-Pr 3-MeO—Ph 228-230 MeCN 348 Me 2-thienyl >300 CHCl₃—EtOH 349 c-Pr 2-thienyl >300 EtOR 350 n-Pr 2-thienyl 292-294 EtOH

TABLE 26 Solvent for recrystalli- Ex. No. R¹ R² M.p. (° C.) zation 351 i-Pr 2-thienyl 293-296 EtOH 352 Me Ph >300 EtOH 353 c-Pr Ph >300 MeCN 354 n-Pr Ph >300 MeCN 355 n-Bu Ph 281-283 MeCN 356 c-hexyll Ph 251-253 MeCN 357 Ph Ph >300 DMF 358 2-Cl—Ph Ph >300 CHCl₃—EtOH 359 3-Cl—Ph Ph >300 DMF 360 4-Cl—Ph Ph >300 DMF 361 2-Me—Ph Ph >300 EtOH 362 3-Me—Ph Ph >300 CHCl₃—EtOH 363 4-Me—Ph Ph >300 CHCl₃—EtOH 364 2-pyridyl Ph >300 CHCl₃—EtOH 365 3-pyridyl Ph >300 CHCl₃—EtOH 366 4-pyridyl Ph >300 CHCl₃—EtOH 367 2-thienyl Ph >300 CHCl₃—EtOH 368 2-furyl Ph >300 EtOH

EXAMPLE 369 Preparation of 1,2-dihydro-5-methyl-2-oxo-1,6-naphthyridine-3-carbonitrile

(1) A mixture of acetylacetone (41 ml), N,N-dimethylformamide dimethylacetal (106.2 ml) and THF (200 ml) was stirred at room temperature for 3 hours. After distilling off the solvent under reduced pressure, the residue was added dropwise to a solution prepared by dissolving metal sodium (13.8 g) in ethanol (600 ml) and adding thereto cyanoacetamide (33.6 g), and the mixture was refluxed for one hour. The reaction mixture was ice-cooled, and the precipitated crystals were separated by filtration. The crystals were dissolved in water (1 liter) and then weakly acidified with 3N hydrochloric acid. The precipitated crystals were separated by filtration and recrystallized from DMF-methanol to give 5-acetyl-6-methyl-1,2-dihydro-2-oxo-3-pyridinecarbonitrile (60 g) as colorless crystals. M.p. 230° C.

(2) A solution of the above carbonitrile (30 g), N,N-dimethylformamide dimethylacetal (25 ml) and DMF (150 ml) was stirred at room temperature overnight. The precipitated crystals were separated by filtration, washed with methanol and then dried. The crystals thus obtained and ammonium acetate (21.9 g) were added to DMF (300 ml), and the mixture was stirred at 130° C. for 3 hours. The reaction mixture was concentrated under reduced pressure, and water was added to the residue, and the resulting crystals were separated by filtration and recrystallized from DMF to give the title compound (Compound No. 1)(25 g) as colorless crystals. M.p. 278° C.

In the same manner as described in Example 369, the corresponding starting materials were reacted to give the compounds of Compound Nos. 2 to 43 as shown in Tables 28 to 29.

TABLE 28

Comp. No. R² M.p. (° C.)  2 H >290  3 Et 268  4 n-Pr 259  5 i-Pr 283  6 c-Pr 276  7 2-Me-c-Pr 242  8 n-Bu 259  9 i-Bu 236 10 c-Bu 270 11 n-pentyl 265 12 c-hexyl >290 13 c-hexyl-CH₂ 269 14 3-c-hexenyl >290 15 Ph >290 16 2-F—Ph >290 17 3-F—Ph >290 18 4-F—Ph >290 19 2-Cl—Ph >290 20 3-Cl—Ph >290 21 4-Cl—Ph >290

TABLE 29 Comp. No. R² M.p. (° C.) 22 2-Br—Ph 291-294 23 3-Br—Ph 280-282 24 4-Br—Ph 25 2-Me—Ph >290 26 3-Me—Ph >290 27 4-Me—Ph >290 28 2-CF₃—Ph 29 3-CF₃—Ph 291-294 30 4-CF₃—Ph >300 31 2-MeO—Ph   289 32 3-MeO—Ph >290 33 4-MeO—Ph   278 34 3-CF₃O—Ph 264-266 35 4-CF₃O—Ph >300 36 1-naphthyl >290 37 2-naphthyl >290 38 2-furyl >290 39 2-thienyl >290 40 5-Cl-2-thienyl >290 41 3-thienyl >290 42 3-pyridyl >290 43 4-pyridyl   285

EXAMPLE 370 Preparation of 1,2-dihydro-5-(2-thienyl)-2-oxo-1,6-naphthyridine-3-carboxylic acid

A mixture of 1,2-dihydro-5-(2-thienyl)-2-oxo-1,6-naphthyridine-3-carbonitrile (10.0 g), ethanol (300 ml) and 10N NaOH (300 ml) was refluxed overnight. After cooling, the reaction mixture was neutralized with acetic acid, and the precipitated crystals were separated by filtration, washed with water, isopropanol and diisopropyl ether in this order and then dried to give the title compound (Compound No. 44)(10.5 g) as pale yellow crystals. M.p. 278° C.

In the same manner as described in Example 370, the corresponding starting materials were reacted to give the compounds of Compound Nos. 45 to 86 as shown in Tables 30 to 31.

TABLE 30

Comp. No. R² M.p. (° C.) 45 H 246 46 Me 273 47 Et 268 48 n-Pr >290 49 i-Pr >290 50 c-Pr >290 51 2-Me-c-Pr 286 52 n-Bu 248 53 i-Bu 259 54 c-Bu >290 55 n-pentyl 263 56 c-hexyl >290 57 c-hexyl-CH₂ >290 58 3-c-hexenyl >290 59 Ph >290 60 2-F—Ph >290 61 3-F—Ph >290 62 4-F—Ph >290 63 2-Cl—Ph >290 64 3-Cl—Ph >290

TABLE 31 Comp. No. R² M.p. (° C.) 65 4-Cl—Ph >290 66 2-Br—Ph 288-291 67 3-Br—Ph >300 68 4-Br—Ph >300 69 2-Me—Ph >290 70 3-Me—Ph >290 71 4-Me—Ph >290 72 2-CF₃—Ph 73 3-CF₃—Ph 275-278 74 4-CF₃—Ph >300 75 2-MeO—Ph   286 76 3-MeO—Ph >290 77 4-MeO—Ph >290 78 3-CF₃O—Ph 265-268 79 4-CF₃O—Ph >300 80 1-naphthyl >290 81 2-naphthyl >290 82 2-furyl >290 83 5-Cl-2-thienyl >290 84 3-thienyl >290 85 3-pyridyl >290 86 4-pyridyl >290

Preparation 1

Capsules:

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5- 5 g (3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one Corn starch 57 g Lactose 10 g Crystalline cellulose 25 g Hydroxypropyl cellulose 2 g Light silicic anhydride 0.5 g Magnesium stearate 0.5 g

According to a conventional method, the above components are mixed and kneaded to give the granules, which are packed into 1000 capsules to give a capsule preparation (each 100 mg).

Preparation 2

Tablets:

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5- 5 g (3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one Corn starch 20 g Lactose 30 g Hydroxypropyl cellulose 5 g Low-substituted hydroxypropyl cellulose 10 g

According to a conventional method, the above components are mixed and kneaded, and thereto are added light silicic anhydride and magnesium stearate, and the mixture is tabletted to give tablets containing 5 mg of the active ingredient in each tablet.

Preparation 3

Powder:

3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5- 5 g (3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one Corn starch 173 g Lactose 300 g Hydroxypropyl cellulose 20 g

According to a conventional method, the above components are mixed and kneaded, pulverized, and thereto is added light silicic anhydride (q.s.) to give 50-trituration.

INDUSTRIAL APPLICATION

The compounds of this invention have high selective affinity to benzodiazepine receptor and are useful as a drug for acting onto benzodiazepine receptor. Although some of the compounds of this invention have agonistic properties, the compounds of this invention are particularly useful as an inverse agonist. The compounds having inverse agonistic properties are expected to be used in clinical fields entirely different from those of agonists, for example, as psychoanaleptic drug or a drug for the treatment of dysmnesia in senile dementia or Alzheimer's disease. 

What is claimed is:
 1. A 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative having the following formula (I):

wherein Het is an oxadiazolyl group, R¹ is a hydrogen atom, a lower alkyl group, a cyclo-lower alkyl group, a trifluoromethyl group, a lower alkenyl group, a lower alkynyl group, a lower alkoxy group, a lower alkoxy-lower alkyl group, a hydroxy-lower alkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group, and R² is a hydrogen atom, a lower alkyl group, a cyclo-lower alkyl group, a cyclo-lower alkylmethyl group, a lower alkenyl group, a cyclo-lower alkenyl group, a lower alkynyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group or a pharmaceutically acceptable acid addition salt thereof.
 2. A 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative according to claim 1, wherein R¹ is a C₁-C₃ alkyl group, a C₃-C₄ cycloalkyl group, or a C₂-C₃ alkenyl group, and R² is a hydrogen atom, a C₁-C₄ alkyl group, a C₃-C₆ cycloalkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heteroaromatic group, or a pharmaceutically acceptable acid addition salt thereof.
 3. A 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative according to claim 1, wherein R¹ is a C₁-C₃ alkyl group or a C₃-C₄ cycloalkyl group and R² is a hydrogen atom, a C₁-C₃ alkyl group, a C₃-C₄ cycloalkyl group, a substituted or unsubstituted phenyl group, or a substituted or unsubstituted heteroaromatic group, or a pharmaceutically acceptable acid addition salt thereof.
 4. A 5-substituted-3-oxadiazolyl-1,6-naphthyridin-2(1H)-one derivative selected from the following compounds or a pharmaceutically acceptable acid addition salt thereof: 3-(5-Ethyl-1,2,4-oxadiazol-3-yl)-5-(2-methylcyclopropyl)-1,6-naphthyridin-2(1H)-one, 3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(2-methylphenyl)-1,6-naphthyridin-2(1H)-one, 3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(4-methoxyphenyl)-1,6-naphthyridin-2(1H)-one, 3-(5-Ethyl-1,2,4-oxadiazol-3-yl)-5-(2-thienyl)-1,6-naphthyridin-2(1H)-one, 3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(4-pyridyl)-1,6-naphthyridin-2(1H)-one, 3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-methyl-1,6-naphthyridin-2(1H)-one, 3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-(3-fluorophenyl)-1,6-naphthyridin-2(1H)-one, 3-(3-Methyl-1,2,4-oxadiazol-5-yl)-5-(3-methylphenyl)-1,6-naphthyridin-2(1H)-one, 3-(3-Methyl-1,2,4-oxadiazol-5-yl)-5-(3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one, 3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-(4-methoxyphenyl)-1,6-naphthyridin-2(1H)-one, 3-(3-Ethyl-1,2,4-oxadiazol-5-yl)-5-(4-pyridyl)-1,6-naphthyridin-2(1H)-one, and 3-(3-Cyclopropyl-1,2,4-oxadiazol-5-yl)-5-(3-thienyl)-1,6-naphthyridin-2(1H)-one.
 5. 3-(5-Methyl-1,2,4-oxadiazol-3-yl)-5-(3-methoxyphenyl)-1,6-naphthyridin-2(1H)-one or a pharmaceutically acceptable acid addition salt thereof.
 6. A pharmaceutical composition for acting on a benzodiazepine receptor, which comprises the compound as set forth in claim 1 in admixture with a pharmaceutically acceptable carrier or diluent.
 7. A drug for acting onto benzodiazepine receptor, which comprises as an active ingredient the compound as set forth in claim
 1. 8. A pharmaceutical composition for acting on a benzodiazepine receptor, which comprises the compound as set forth in claim 4 in admixture with a pharmaceutically acceptable carrier or diluent.
 9. A pharmaceutical composition for acting on a benzodiazepine receptor, which comprises the compound as set forth in claim 5 in admixture with a pharmaceutically acceptable carrier or diluent.
 10. A drug for acting on a benzodiazepine receptor, which comprises as an active ingredient the compound as set forth in claim
 4. 11. A drug for acting on a benzodiazepine receptor, which comprises as an active ingredient the compound as set forth in claim
 5. 